1. Field of the Invention
The present invention relates to an engine mounting structure in a motorcycle.
2. Description of the Background Art
In a certain motorcycle, an engine is mounted to a body frame and a rear fork is vertically swingably attached to the engine. As this type of a motorcycle there is known, for example, xe2x80x9cStep Device in a Sitting Type Motorcyclexe2x80x9d (hereinafter referred to as the xe2x80x9cbackground artxe2x80x9d) disclosed in Japanese Examined Patent Publication No. Hei 1-33392.
According to the background art, as shown in FIG. 1 of the aforesaid publication, an engine E is mounted to a vehicle body 1 (this and the following reference numerals and marks are quoted from the aforesaid publication). A rear fork F is vertically swingably attached to a crankcase 18 of the engine E. A rear wheel Wr is rotatably secured to a rear portion of the rear fork F.
Further, in the background art, as shown in FIG. 2 of the aforesaid publication, an output shaft 19 is disposed behind a crankshaft 9 for transmitting power of the crankshaft 9 to a belt type continuously variable transmission M. A swing center of the rear fork F is aligned with the axis of the output shaft 19.
In the background art referred to above, the engine E, which is highly rigid, is fixed to the vehicle body 1 with bolts, thereby enhancing the rigidity of the vehicle body 1 to a greater extent. However, with travel of the motorcycle, the rear wheel Wr vibrates according to road surface conditions and this vibration (hereinafter xe2x80x9crunning vibrationxe2x80x9d) is transmitted directly from the rear wheel Wr to the vehicle body 1 through the rear fork F and the engine E. Therefore, there is room for improvement in damping the running vibration transmitted to the vehicle body 1 and thereby enhancing the comfortableness in riding the motorcycle.
It is therefore an object of the present invention to provide a technique for enhancing the comfortableness in riding a motorcycle wherein a rear fork for supporting a rear wheel is swingably attached to a crankcase of an engine.
According to the invention, in a motorcycle wherein a front portion of a rear fork which supports a rear wheel is vertically swingably attached to a crankcase of an engine, there is provided an engine mounting structure characterized in that the engine is mounted to a body frame through elastic members. With travel of the motorcycle, the running vibration is damped by the elastic members and is therefore unlikely to be transmitted from the engine to the body frame.
According to the invention, a cylinder extends forward from the crankcase of the engine, the body frame is provided with a rising frame member which is substantially orthogonal to the cylinder in side view, and the elastic members are attached to the rising frame member in higher and lower positions respectively than the cylinder.
By extending the cylinder forward from the crankcase, spaces are formed above and below the crankcase. By utilizing these upper and lower spaces, elastic members are disposed in higher and lower positions respectively than the cylinder and are attached to a rising frame which is substantially orthogonal to the cylinder in side view. It is possible to ensure a sufficient distance between the upper and lower elastic members.
With running vibration, a moment acts on the case (e.g., crankcase) of the engine, centered on the upper or lower elastic member. The longer the distance between the upper and lower elastic members, the smaller the force based on the moment and acting on the upper or lower elastic member. Since the distance between the upper and lower elastic members can be set relatively large, it is possible to decrease the force acting on the upper or lower elastic member. As a result, the vibration absorbing performance of the elastic members is enhanced. The elastic members are disposed in higher and lower positions respectively than a swing center of the rear fork.
The swing center of the rear fork is aligned with the axis of a crankshaft of the engine. A swing angle of a mechanism (a power transfer mechanism) for transmitting power from the crankshaft to the rear wheel coincides with a swing angle of the rear fork. Consequently, the construction of the power transfer mechanism becomes simpler.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.